add tickbox to ignore modifications, or export both a modified and unmodified version of the strand, when exporting DNA sequences

[dave-doty]

It is sometimes the case that someone wants to have two versions of a strand: one with a modification (e.g., 5' biotin) and one without. Support this directly when exporting DNA sequences by allowing a tickbox that ignores modifications, and another that outputs two versions of the strand: one with modifications and one without.

See also https://github.com/UC-Davis-molecular-computing/scadnano-python-package/issues/148